How to initialize a database on a data volume container? - postgresql

Question

Here my simple scenario, I have a simple Flaskapp that connect to a postgres this way:
SQLALCHEMY_DATABASE_URI='postgresql://username:secretpassword#postgres:5432/myproj'
And I have a simple docker-compose.yml:
version: '2'
services:
postgres:
image: postgres:latest
volumes_from:
- data
environment:
POSTGRES_PASSWORD: secretpassword
POSTGRES_USER: username
POSTGRES_DB: myproj
ports:
- "5432:5432"
web:
build: .
volumes_from:
- app
ports:
- "5000:5000"
depends_on:
- postgres
data:
image: postgres:latest
volumes:
- /var/lib/postgresql/data
command: "true"
app:
build: .
volumes:
- .:/myproj
command: "true"
I need to lunch a made by myself flask script, that creates the tables for my app:
export FLASK_APP='./myproj/__init__.py'
flask createdbs
I have put these 2 operation in the Dockerfile of my web service but because my service and the postgres service have a depends_on relationship, the postgres db host is not available during the building phase.
Any suggestion on the best way to achieve this ? I want to avoid hacks, I would prefer respect a correct Docker workflow.

Answer 1

One way to do it is to use the "command" keyword:
https://docs.docker.com/compose/compose-file/#/command
(look also at entrypoint keyword)
web:
build: .
volumes_from:
- app
ports:
- "5000:5000"
depends_on:
- postgres
command: "export FLASK_APP='./myproj/__init__.py' && flask createdbs"
or using command just to launch your flask script and let your export in your dockerfile.
Note that "depends_on" only start one container before the other, but do not wait your postgres database to be ready. If you want to wait until postgres is ready to answer, you can use scripts like "wait-for-it.sh postgres:5432" that are well explained in docker-compose doc: https://docs.docker.com/compose/startup-order/